A switch of this kind is known from EP 0 858 090 A2.
The known switch comprises a two-part housing made from insulating material, into which housing a temperature-dependent switching mechanism is inserted. The switching mechanism comprises a spring part with a spring disc as a free end, which spring disk carries approximately centrally a moving contact part, on which moving contact part a bimetallic snap-action disc is also arranged. As fixed end of the spring part a holding attachment is provided laterally at the spring disc and is mounted by means of a pin on a bottom electrode, which is provided on the lower part of the housing.
The moving contact part interacts with a fixed opposing contact which is provided internally on the cover part of the housing, as a cover electrode. The bottom electrode and the cover electrode each have an external connection, into which the stripped end of the connecting wire is inserted.
Via its rim area, the spring disc rests on an inner, projecting shoulder, which shoulder is provided internally on the housing lower part; the rim area not being equipped with said holding attachment.
Depending on the temperature of the bimetallic snap-action disc, the moving contact part rests on the fixed opposing contact, thus resulting in an electrically conductive connection being made between the two external connections via the fixed opposing contact, the moving contact part, the spring disc and the holding attachment.
When the temperature of the bimetallic snap-action disc rises above the response temperature thereof, then it switches over from its convex shape to a concave shape, in which its rim is supported on shoulders and stops which are provided for this purpose in the upper housing part, thereby lifting the moving contact part off the fixed opposing contact, against the force of the spring disc. To do this, it is necessary for the bimetallic snap-action disc to push the spring disc through, as a result of which the spring disk also changes from its convex shape to the concave shape.
In the known switch, the temperature-dependent switching mechanism is a captive unit comprising the spring part, which is formed by the spring disc and the holding attachment, the bimetallic snap-action disc and the moving contact part.
AT 307 770 B discloses a temperature-dependent switch in which a housing lower part which is in the form of a pot and is composed of metal is closed by a cover composed of insulating material. A bimetallic switching mechanism comprising a bimetallic snap-action disc and a spring part is arranged in the housing that is formed in this way, which bimetallic switching mechanism has a circular frame and a spring tongue which extends inwards from this frame and at whose free end a moving contact part is arranged. The moving contact part interacts with a fixed opposing contact, which is arranged centrally on the cover part. The bimetallic snap-action disc is firmly connected to the spring tongue and lifts the moving contact part off the fixed opposing contact, with which it is otherwise in contact, when the response temperature is exceeded.
DE 101 19 467 A1 discloses a temperature-dependent switch having a plastic carrier forming an accommodation area for a switching mechanism comprised of a spring and a bimetallic tongue. At the upper and lower sides of the carrier, the accommodation area is closed off by each a metal plate, an external connection being provided at each metal plate.
At its first end, the spring is clamped to the plastic carrier, whereby the bimetallic tongue is clamped to the spring.
At one of the metal plates closing off the accommodation area a fixed counter contact is provided, which counter contact cooperates with a moveable contact provided at the front free end of the spring. Thus, when the known switch is closed, the current to be switched flows through the spring.
When assembling the known switch, many parts have to be put together. Only when completely assembled, the spring of the known switch is in firm contact with one of the metal plates.
The temperature-dependent switch and switching mechanisms which have been described so far are used to protect an electrical appliance against an excessively high temperature. For this purpose, the supply current for the appliance to be protected is passed through the temperature-dependent switch and the temperature-dependent switching mechanism, respectively, with the switch and the switching mechanism, respectively, being thermally coupled to the appliance to be protected. At a response temperature, which is predetermined by the snap-over temperature of the bimetallic snap-action disc, the respective switching mechanism then opens the circuit, by lifting the moving contact part off the fixed opposing contact.
All of the switching mechanisms described so far have the advantage that the current to be disconnected does not flow directly via the bimetallic snap-action disc but is passed via the spring part. This reduces the intrinsic heating of the bimetallic snap-action disc, although heat is still created in the interior of the switches as a result of the intrinsic heating of the spring part, as a result of which this intrinsic heating also influences the switching response, in addition to the externally supplied heat in the appliance to be protected.
While the intrinsic heating of the spring part is undesirable in the switches and switching mechanisms which have been described so far, switches are also known in which a series resistance is also provided, which series resistor is heated in a defined manner by the current flow of the appliance to be protected. If the current flow is too high, this resistance is heated to such an extent that the snap-over temperature of the bimetallic snap-action disc is reached. In addition to monitoring the temperature of the appliance, this also makes it possible to monitor the current flowing, and the switch therefore has a defined current dependency.
In order to ensure that a switch of this kind does not close again after the appliance or the series resistance has cooled down, it is also known for a further resistance, preferably a PTC thermistor, to be provided in parallel with the temperature-dependent switching mechanism, which further resistance is electrically short-circuited by this temperature-dependent switching mechanism when the latter is closed. However, when the switching mechanism opens, the parallel resistance carries a portion of the previously flowing current and in the process is heated to such an extent that it produces a sufficient amount of heat in order to maintain the bimetallic snap-action disc at a temperature which is above the response temperature. This process is referred to as self-holding and prevents a temperature-dependent switch from closing again in an uncontrolled manner when the appliance to be protected has cooled down again as a result of the current being disconnected.
In the case of the switches and switching mechanisms described so far, the series resistance and the parallel resistance must be provided separately, which is associated with corresponding complexity.
However, on the other hand, these designs are also subject to the disadvantage that they are complicated and are formed from a large number of components, with a high level of production precision being required, in particular because of the round spring discs, because the round discs require a high-precision contact surface on the respective rim. For this reason, the housings are frequently manufactured as turned part, in order to ensure high-precision contact surfaces. The small contact surfaces between the spring discs and the contact rims in this case lead, however, to undesirably high contact resistances, with the spring parts themselves frequently being undesirably heated to a major extent, as a result of which a certain current dependency is present.
A further disadvantage of these switches is that they can switch only low current levels. Higher currents would lead to the formation of arcs and to the generation of sparks on opening of the switch, which leads to undesirable contact erosion and, if the contact separation is too small in the open state, even to the arc not being quenched or not being quenched quickly, as a result of which an undesirably high residual current can still flow. Furthermore, in this case, there is a risk of the flying sparks damaging the bimetallic snap-action disc or leading to rapid ageing of the bimetallic snap-action disc, both of which may result in an undesirable shift in the switching point.
In addition, the connection between the spring part and the first external connection does not allow high currents, because the riveting or clamped connection which is possible by virtue of the design, between the external connection and the fixed end of the spring part, leads to connections which are not still safe at high currents as well, because of the remaining contact resistance. In fact, flashovers occur to areas of the connected metal parts which are not in close contact or tightly fitting over the entire area, thus leading to contact erosion and, in consequence, to further deteriorating contact resistances during the course of operation.
In order to switch relatively high currents up to, for example, 10 amperes, temperature-dependent switches with a contact bridge are therefore used, as are described in DE 26 44 411 A1 and DE 197 08 436 A1. These switches have two fixed opposing contacts and two moving contact parts, which are connected to one another via a contact bridge. The bimetallic snap-action disc is arranged on the side of the contact bridge facing away from the fixed opposing contacts, and is therefore protected against possible sparking.
However, these switches have a complicated design and are difficult to assemble. A further disadvantage is that two pairs of switching contacts comprising a contact part and an opposing contact are required. Since these contact parts and opposing contact parts must be very massy because of the high switching currents and can be manufactured only as expensive turned parts, the known switches are very costly also for this reason. Furthermore, during assembly, care must be taken to ensure that the two moving contact parts are located precisely opposite the fixed opposing contacts. This also leads to stringent requirements both with regard to dimensional accuracy of the various individual parts, and with regard to the assembly quality itself.